Nickel containing hydrogenases

Abstract

Hydrogenases have been purified from different biological sources. They are highly diversified enzymes in terms of active centers constitution, although they catalyze the simplest oxidation-reduction process: H 2 ⇌ 2H + + 2e. Hydrogenases have been recognized so far to be ironsulfur proteins. Generally they contain from four to twelve atoms of non-haem iron arranged in FeS clusters representative of the known basic structures, e.g., [2Fe-2S], 3Fe-xS], and [4Fe-4S] [1–7]. Recently, nickel joined the group of transition metals relevant in biological oxidation-reduction ▪ processes. It was shown to be a structural component of the hydrogenases isolated from Desulfovibrio gigas [1, 2], Desulfovibrio desulfuricans (ATTC 27774) [3], Desulfovibrio desulfuricans (Norway strain) [4], Methanosarcina barkeri [5], Methanobacterium thermoautotrophicum [6] and Chromatium vinosum [8]. With the exception of the last one, they were demonstrated to contain EPR nickel redox dependent signals. As an example, D. gigas hydrogenase exhibits rhombic EPR signals, with g-values 2.31, 2.23 and 2.02 (see Fig. 1). Using isotopic reconstitution by 61Ni (nuclear magnetic moment I = 3 2 ), the EPR signal was proven arise from a nickel species [9]. The same types of experiments were reported for M. thermoautotrophicum [6] and D. desulfuricans (ATCC 27774) hydrogenases [3]. A detailed EPR study on the oxidation-reduction transition of the EPR detectable species in the presence of reductant (dithionite and hydrogen) indicates [1, 2]: 1. The reduction of the Ni EPR active species is an one-electron process (possibly associated with the redox couple Ni(III)Ni(II)). 2. No evidence was found so far for exchangeable protons in the vicinity of the nickel center in the oxidized (native) state. However, hydrogen reduced samples originate a different EPR rhombic Ni signal, which may represent an active transient species occurring during the activation of hydrogen molecules [9]. Thus, it is attractive to propose the presence of a hydride intermediate in analogy with nickel catalysts involved in hydrogenation processes [10]. 3. Although the determined mid-point redox potential (−220 mV) is more negative than that expected for nickel compounds [11] it is still more positive than that of the substrate couple H 2/H +. The value determined was shown to be pH dependent [2].